1. Field of the Invention
The invention concerns: a method to correct artifacts in magnetic resonance (MR) images by means of an MR pulse sequence in which gradients are switched (activated) simultaneously during the radiation of at least one excitation pulse, as well as a magnetic resonance apparatus and an electronically readable data medium for implementing such a method.
2. Description of the Prior Art
The magnetic resonance modality (also known as magnetic resonance tomography) is a known technique with which images of the inside of an examination subject can be generated. Expressed simply, for this purpose the examination subject is positioned within a strong, static, homogeneous basic magnetic field (also called a B0 field) having a field strength of 0.2 Tesla to 7 Tesla and more, such that the nuclear spins of the examination subject are oriented along the basic magnetic field. To trigger nuclear magnetic resonance signals, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, the triggered magnetic resonance signals are measured (detected) in a form known as k-space data, and MR images are reconstructed, or spectroscopy data are determined, based on these nuclear magnetic resonance signals. For spatial coding of the measurement data, rapidly switched magnetic gradient fields (also shortened to “gradients”) are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with such values, for example by means of a multidimensional Fourier transformation.
Sequences with very short echo times TE, for instance TE less than 0.5 milliseconds, offer new fields of application for magnetic resonance tomography. They enable the depiction of substances that cannot be shown with conventional sequences such as (T)SE ((Turbo)Spin Echo) or GRE (Gradient Echo), since the respective decay time of the transverse magnetization T2 in such ultrashort sequences is markedly shorter than the possible echo times of the conventional sequences, which means that in the conventional sequences the detectable signal has already decayed at the acquisition point in time. In contrast, with echo times in the same time range of these decay times, it is possible to show the signals of these substances, for example in an MR image. The decay times T2 of teeth, bones or ice lie between 30 and 80 microseconds, for example.
The application of sequences with ultra-short echo times (UEZ sequences) thus enables bone and/or teeth imaging and/or the depiction of cryo-ablations by means of MR, for example, and can be used for MR-PET (combination of MR and positron emission tomography, PET) or PET attenuation correction.
Examples of UEZ sequences are UTE (“Ultrashort Echo Time”), for example as it is described in the article by Sonia Nielles-Vallespin, “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle”, Magn. Res. Med. 2007; 57; P. 74-81; PETRA (“Pointwise Encoding Time reduction with Radial Acquisition”) as is described by Grodzki et al. in “Ultra short Echo Time Imaging using Pointwise Encoding Time reduction with Radial Acquisition (PETRA)”, Proc. Intl. Soc. Mag. Reson. Med. 19 (2011) P. 2815; or z-TE as is described by Weiger et al. in “MRI with zero echo time: hard versus sweep pulse excitation”, Magn. Reson. Med. 66 (2011) P. 379-389.
Generally, in these sequences, a hard delta pulse is applied as a radio-frequency excitation pulse, and the data acquisition is subsequently started. In PETRA or z-TE, the gradients are already activated during the excitation. The spectral profile of the excitation pulse corresponds approximately to a sinc function. In the case of insufficient pulse bandwidth or gradients that are too strong, it may be that the outer image regions are no longer sufficiently excited.
In the reconstructed MR image, this incorrect excitation has the effect of blurring artifacts at the image edge, which are pronounced more strongly the stronger the gradients switched during the excitation.
An insufficient excitation thus leads to artifact-plagued MR images. This problem has previously for the most part been ignored. At best it is attempted to optimally reduce the strength of the gradients. However, imaging-relevant variables such as the readout bandwidth, the repetition time TR and the contrast of the image therefore change. For example, a reduction of the gradient strength increases the minimum necessary repetition time TR, and therefore also the total measurement time. Furthermore, such artifacts could be avoided in that the excitation pulses are selected to be particularly short in order to increase the excitation width. However, at the same time the maximum possible flip angle and the precision of the actually sent RF excitation pulse are therefore proportional to the duration of the RF excitation pulse. For example, given a duration of the excitation pulse of 14 microseconds the maximum flip angle amounts to approximately 9°, and given a reduced duration of the excitation pulse to 7 microseconds the maximum flip angle would amount to only approximately 4.5°. This procedure therefore also cannot be used without limitations and is accompanied by a degradation of the image quality.